Answer: approximately nine weeks.
I would have never discovered this (or bothered to count) if not for the fact that just after Christmas I went hunting for cat food and (for reasons too inane to recount) came back with a twenty-two pound bag of kibble -- the combined weight of both of my cats.
There was something very strange about measuring cat food in cat-weight. Almost like I was buying cats for my cats to eat.
Nine weeks later the strangeness has ended, the kibble is gone, and in another day's time it will find itself completely processed. In the meantime, I have to go shopping.